GaN (gallium nitride) compound semiconductors have already been put into practical use in, for example, blue light-emitting diodes (LEDs). In general, a GaN compound semiconductor is grown, on a substrate formed of sapphire (i.e., a lattice mismatching material), through metal organic chemical vapor deposition (MOCVD) by use of an organometallic compound serving as a Group III element source and ammonia (NH3) serving as a Group V element source. However, in the case where a GaN semiconductor layer is formed directly on a sapphire substrate, blue light of high emission intensity fails to be obtained, since the GaN semiconductor layer exhibits very poor crystallinity and surface morphology. In an attempt to solve problems involved in such lattice mismatching semiconductor growth, there has been proposed a technique for growing an AlN buffer layer at a low temperature of about 400° C. between a sapphire substrate and a GaN semiconductor layer. According to this technique, the GaN semiconductor layer, which is grown on the buffer layer, is improved in crystallinity and surface morphology (Japanese Patent Application Laid-Open (kokai) No. 2-229476). Meanwhile, an attempt has been made to develop a technique which does not employ the aforementioned low-temperature buffer layer growth technique (International Publication No. 02/17369 Pamphlet).
However, even when a GaN semiconductor layer is formed through such a technique, the semiconductor layer exhibits a dislocation density of 108 to 1010 cm−2. Dislocations are considered to serve as nuclei for non-radiative recombination. Particularly, in the case of an ultraviolet LED, which emits short-wavelength light (wavelength of 380 nm or less), dislocations greatly affect emission efficiency, and therefore the dislocation density must be reduced. In an ultraviolet LED, light whose energy is nearly equal to the bandgap of GaN (3.4 eV) is emitted from the light-emitting layer, and thus a considerable amount of the emitted light is absorbed by a GaN semiconductor layer which underlies the light-emitting layer; i.e., the GaN semiconductor layer exhibits the light absorption effect. Suppression of such light absorption requires a technique for lamination of a thick layer of an aluminum gallium nitride (AlGaN) semiconductor, which has a larger bandgap.
In the case of an AlGaN semiconductor layer, difficulty is encountered in growing crystals of high quality, as compared with the case of a GaN semiconductor layer, which is generally employed in, for example, a blue LED. Therefore, the crystallinity of the AlGaN semiconductor layer is lower than that of the GaN semiconductor layer. When an Al-containing Group III nitride semiconductor underlying layer is formed on a substrate, misfit dislocations are generated as a result of the difference in lattice constant between the substrate and the underlying layer, and the thus-generated misfit dislocations thread through the underlying layer and reach the surface thereof. Therefore, high-density dislocations, attributed to the misfit dislocations, are generated in Group III nitride semiconductor layers which are to be provided on the Group III nitride semiconductor underlying layer formed on the substrate. In order to suppress generation of such misfit dislocations in the Al-containing Group III nitride semiconductor layer (e.g., an AlGaN semiconductor layer), in general, a low-temperature buffer layer is formed between the substrate and the Group III nitride semiconductor layer through the above-described low-temperature buffer layer growth technique, thereby reducing the effect caused by the aforementioned lattice constant difference (e.g., Japanese Patent Application Laid-Open (kokai) No. 6-196757).
However, even in the case where such a low-temperature buffer layer is provided, the resultant Al-containing Group III nitride semiconductor layers, which constitute a semiconductor element, exhibit a high dislocation density (about 1010 cm−2); i.e., low crystallinity. Therefore, when a semiconductor light-emitting device (i.e., an ultraviolet LED) is produced from the semiconductor element, the resultant semiconductor light-emitting device exhibits lowered emission efficiency; i.e., the device fails to attain intended characteristics.
In view of the foregoing, there has been proposed a technique in which a thick GaN layer (thickness: about 8 μm) is formed, at a high temperature, atop a sapphire substrate via a low-temperature buffer layer, and an AlGaN layer is grown on the GaN layer (Ito, et al., “PREPARATION OF AlxGa1-xN/GaN HETEROSTRUCTURE BY MOVPE,” J. Cryst. Growth, 104 (1990), 533-538). In connection with this technique, there has been proposed a technique in which the thick GaN layer and the sapphire substrate are removed through polishing after growth of the AlGaN layer, to thereby form a GaN-free AlGaN layer (Morita, et al., “High Output Power 365 nm Ultraviolet Light-Emitting Diode of GaN-Free Structure,” Jpn. J. Appl. Phys. Vol. 41 (2002), 1434-1436).
However, in the case where an AlGaN layer is grown on a thick GaN layer, since the lattice constant of GaN differs from that of AlGaN, once the elastic limit has been surpassed, cracking occurs in the AlGaN layer. Therefore, difficulty is encountered in growing crack-free crystals of high quality. Particularly when the mol fraction of AlN in the AlGaN layer or the thickness of the AlGaN layer is increased, such cracking may become considerable. When an LED is produced from the thus-formed semiconductor structure, the light absorption effect of the thick GaN layer would cause problems. Meanwhile, removal of the thick GaN layer and the sapphire substrate after growth of the AlGaN layer leads to very poor productivity.